One of many problems faced by governments today are illegal attempts to introduce nuclear materials within their borders. In order to adequately guard against the smuggling of such materials, there is a need to provide portable radiation detectors that exhibit a high degree of detection and isotopic identification capability, reliability, and ruggedness for field use.
The core technology behind the present invention is that of a high-purity germanium (HPGe) detector (semiconductor diode detector). As used herein, the high-purity classification means an electrically active impurity level less than 1 impurity in 2×1010 germanium atoms. There are two primary historical threads related to the HPGe detector. The first is the detection of gamma rays of specific energy and the second is the cooling mechanism needed to allow the detector to perform properly. The objective in any radioisotope identification system is to identify the presence of one or more specific isotopes by detecting the characteristic gamma rays emitted by the isotopes. Most radioisotopes emit gamma rays (some 7,000 isotopes emitting roughly 100,000 gamma rays of unique energy) and each isotope has a characteristic “fingerprint” of one or more discrete energy gamma rays.
In view of the large number of discrete gamma rays given by nature, the better a detector can discriminate one energy from another, the better the detector is able to discriminate one isotope from another. This ability is referred to as the energy resolution of a detector and can be characterized as dE/E, i.e., the width (dE) of a peak in a measured spectrum from a discrete, isolated gamma-ray of energy E. When the quantities dE and E are both expressed in the same energy units then the ratio can be quoted as a percentage. The smaller the value of dE/E, the better the detector is at determining one isotope from another.
There are three main classes of gamma-ray detectors that exhibit the ability to determine energy resolution. The first, inorganic scintillators, has been available since the 1940's. The prime example of this class is thallium doped sodium iodide, NaI(TI). This class has the advantages of being easily fabricated in arbitrarily large crystals, operating at room temperature, exhibiting high intrinsic stopping power, relatively inexpensive, and extremely rugged. The disadvantage is that sodium iodide has a relatively low energy resolution of dE/E˜8–10%, and the best inorganic scintillators are only somewhat better.
The second class of detectors is large band gap or so-called room temperature semiconductors. This is a relatively new class of detectors. They have significantly better energy resolution than the inorganic scintillators, having dE/E˜2–3%. However, these detectors are presently limited in size to about ˜1 cm3. Since the ability to detect a gamma ray is scaled as the efficiency divided by the energy resolution, having such small detector sizes limits their usefulness, even though the energy resolution is much better compared to inorganic scintillators.
The third class of detectors is the HPGe detector. HPGe detectors were first fabricated in the 1970's. HPGe detectors have excellent energy resolution, typically dE/E˜0.1–0.2%. HPGe detectors can also be made fairly large, exhibiting diameters of up to 12 cm and lengths of 11 cm, with a concomitant increase in detection capability. The primary disadvantage is that they must be cooled to around 80° K in order to work properly. In order to reach and maintain such a low temperature as 77° K, liquid nitrogen has historically been the cryogen of choice. However, use of liquid nitrogen precludes sustained use in portable handheld detectors as liquid nitrogen requires a complicated and bulky support system.
Roughly 20 years ago, mechanical cryocoolers of various designs capable of reaching 77° K without using liquid nitrogen entered the market. These were typically large, bulky units (10's of kg) with power consumption in the hundreds of Watts. In the early 1990's Hymatic, Inc., a U.K. defense contractor, licensed from Oxford University the design for a relatively small (˜1 kg), low power (˜10 W), rugged-mechanical cryocooler capable of cooling to 77° K. One of the first uses of this type of cooler was to cool HPGe detectors mounted on satellites in the mid 1990's. The present invention exemplifies the next step in HPGe evolution: utilizing a cryocooler in a handheld gamma ray detector instrument exhibiting exceptional energy resolutions
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.